Thermistor Question

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Hi all
         Can someone help me with a thermistor question, I have a USB dual
hard drive enclosure with two 1TB HD in it. It's a Ritmo and has a cooling
fan that runs (noisily) all the time.
I know it's 12v and draws 110mA and I want to put a NPT thermistor in the
box so the fan only runs when the HD's get warm to hot.
I see the thermisters are rated xx ohms @25C and I'm wondering how many ohm
thermistor I need to stop the fan on start up, that will start the fan as
the temp increases.
This is the link from a nearby supplier I might use-
http://www.jaycar.com.au/products_uploaded/RN3430.pdf
Thanks Gaz



Re: Thermistor Question



"Gaz"
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** Have you considered that the current flowing through the NTC to the fan
will HEAT the NTC ?

 Means once the fan is on, it will likely stay on permanently.

 My advice is to leave well alone.



.....  Phil




Re: Thermistor Question


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If he ever manages to get it to turn on in the first place.
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--
Clint Sharp

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Annoying isn't it.
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You mean NTC I think? NPT is a type of thermistor fitting I think.
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That's not quite the way it works, most thermally controlled fans that I
have seen have the thermistor integrated into the motor control logic of
the fan.

  Third party thermal fan controllers use the thermistor to control a
power transistor or IC that either varies a PWM signal or the voltage
applied to the fan (some fans respond better to voltage changes than PWM
and vice versa). You might strike lucky but I suspect it won't be
reliable and you're going to need to do some experimentation to get it
right.

Probably cheaper and better to buy a ready built fan controller or a fan
that is designed with a thermistor in place already.  For the (rapidly
diminishing) cost of the two drives, I'd just buy something.

There are plenty of plans out there for thermal fan controllers if you
really want to build your own.
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--
Clint Sharp

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Probably cheaper and easier to buy a quiet fan.

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You are better off trying to slow the fan, in my experience you don't
need to knock off a lot of fan speed to achieve a significant noise
reduction.

you could try putting diode(s) in series with the +12v line, each
should knock off about .5v or so.

I also discovered a couple of years back (at least with an 65W AMD
Athlon CPU) that by slowing the processor fan from 2000 RPM to 1000
RPM in the BIOS made no difference to the CPU temperature, but a lot
to the noise factor.  Of course, if it was a higher wattage processor
it might not be as effective at reducing heat but I didn't check this.

Other option is a quieter fan.
Recently we got some Zalman units that when run on the bench, even at
full  +12v (and therefore max speed?) were hardly audible.  These were
6" though, and a hard drive enclosure probably has smaller fans. These
Zalman units also had an adaptor included that had a 220 ohm 1w
resistor in series with the +12v. This would reduce the speed quite a
bit if plugged in series with the fan connector.

You could try to slow the fan, and check the drive temperature while
doing this to see if a happy medium can be reached.  If the drive gets
too hot, then unfortunately you are stuck with full speed and full
noise.

Make sure that you always have the drive where it will get plenty of
airflow, not in a cupboard or on a desk with stuff stacked around or
on it.

Re: Thermistor Question


finger to keyboard and composed:

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The HDs will become hot much sooner than the box will.

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How are you going to detect the temperatures of the hard drive HDAs?
What happens if one drive runs hotter than the other? What happens if
one drive doesn't spin up and contributes nothing to the heat load?

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

Re: Thermistor Question



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###Yes they will, but they are both crammed into this box with little room,
so the box would also heat up fairly quickly, so a thermister mounted in the
box will soon detect a temperature rise.


What happens if one drive runs hotter than the other.
 one drive doesn't spin up and contributes nothing to the heat load?

###These HD's hold movies for a media player and as such, only one is in use
at a time.

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